Over the past few years, solid state detectors such as silicon strip detectors have revolutionized high energy and nuclear physics research. The progress and demand for silicon strip detectors also increased in other fields where their potential high resolution detection capability became apparent. Although an excellent detector, silicon, with an atomic number (Z) of 14, does not have good quantum efficiency for higher energy x-rays and gamma rays. Therefore, recently a significant amount of research has been carried out to develop high-Z strip and pixel detectors. Out of this work, six detector materials have become the potential front runners, Germanium (needs cryogenic cooling), CdZnTe, CdTe, HgI2 and GaAs (both can be used at or near room temperature). A newcomer to the field, with very high Z, is PbI2. These materials provide high detection efficiency for x-ray energies in the 10 to 1,000 keV range with detector thickness of about 0.5 to 15 mm. One positive effect of this small thickness is that depth effects, which degrade position resolution for radiation coming in at an angle, are minimized. Consequently, these high-Z detectors are now routinely manufactured with strip or pixel sizes in the mm to sub-mm-range. Such high spatial and energy resolution two-dimensional x-ray and gamma ray sensors are expected to become the standard in the future.
Although strongly promising high-Z position sensitive solid state detectors were developed, an essential component to make them viable instruments for detecting and imaging x-rays, gamma rays and particles has been missing. Such detectors have many channels with small pitch, and reading them out with conventional discrete or hybrid electronics is not a viable option. These detectors require monolithic multichannel readout electronics to handle both the high number of channels and small pitch. Such ASIC chips, e.g., the Amplex (CERN) and SVX (LBNL) chips, have been developed for accelerator-based high energy physics experiments. However, these chips lack two major functions, which are not needed for those experiments but render the chips mostly unsuitable for use in nuclear physics, astrophysics, and medical and industrial imaging:
1. They do not have a self trigger output. In high energy physics experiments, an external machine trigger is available to inform the data acquisition (DAQ) system about the exact time of an event for reading out the chips. In addition, the event trigger is typically based on the overall event topology rather than the signal levels in individual channels, which precludes its implementation on the readout chip.
2. The solid-state detectors for which these ASICs were developed provide position information only; the energy information is largely irrelevant as the particles of interest are all minimum ionizing anyway. Consequently, such chips do not need to have low noise and thus high energy resolution capability.
By contrast, in space-based (high-energy) astrophysics as well as most medical and industrial imaging, the x-ray and gamma-ray photons and charged particles come randomly. In many applications, it is also important to measure the x-ray, gamma ray and particle energies with as high accuracy as possible. Therefore, the application of position sensitive solid state detectors to nuclear and astrophysics and to medical and industrial imaging was largely delayed as a suitable ASIC readout chip was not available. There have only been few exceptions such as the ACE chip used with silicon strip detectors on board the Advanced Composition Explorer (ACE) space mission. It is thus important to develop versatile ASICs for reading out solid state sensors for application to the above mentioned fields.
Previously we have developed a chip, called RENA (Readout Electronics for Nuclear Application) for a new scintimammography system. The RENA chip has been patented (U.S. Pat. Nos. 5,696,458, 6,150,849 and 6,333,648, incorporated herein by reference). This chip has reached a level where it was useful for imaging as well as physics research applications using various kinds of solid-state detectors; for example, it has been used successfully with silicon strip and CdZnTe pad detectors. The RENA chip is a 32-channel, mixed signal, low-noise, general purpose monolithic application specific integrated circuit (ASIC). It was developed as the front-end electronics chip for medical imaging such as gamma camera and SPECT (Kravis et al., 1999). Its dynamic range is 50,000 electrons. The chip has a self-trigger output so that random signals without an external trigger can be processed. It offers several different externally selectable integration (peaking) times to accommodate different charge collection times for different detectors. It has several readout and data acquisition modes for versatile implementation and for detailed diagnostic testing. The output signals from the 32 channels are multiplexed to a single analog output buffer under the control of the chip's readout section. Significant effort was spent to make RENA low noise (≈150 e rms @ 0 pF input capacitance), but tests performed have indicated there are new ways to improve the noise. Also the RENA chip could only partially answer the requirements of many applications listed above. Therefore, a new ASIC, RENA-2, is developed, which can have different dynamic ranges and shaping (peaking) times, fast timing, low power consumption, lower noise, simplified user interface, and reduced channel-to-channel mismatch of the trigger levels, etc.
We describe here the new RENA-2 front-end readout ASIC designed to address these concerns and also bring significantly more functionality. The new chip is designed to be versatile and, therefore, easy to be modified and optimized for different applications, have much lower noise and thus much improved energy resolution, enabling users to take advantage of the exceptional potential for high energy resolution that solid-state detectors offer. Below the design and specifications of the new ASIC will be discussed in detail.